Adds a NIX_TIMEOUT environment variable which can be set to a number
of seconds that is the maximum allowed time each Nix builder can run.
By default this is set to 60 seconds, which should be plenty for most
use-cases as Nixery is not expected to be performing builds of
uncached binaries in most production cases.
Currently the errors Nix throws on a build timeout are not separated
from other types of errors, meaning that users will see a generic 500
server error in case of a timeout.
This fixes#47
